U.S. Pat. No. 3,713,500 relates to the alteration of the orientation of a drilling assembly by arranging for the drilling head to be rotatable relative to the end of a drill pipe. U.S. Pat. No. 3,841,420 describes means for holding the drilling assembly against the drill rotation torque by the use of a clutch mechanism or a torque balancing force, and to avoid having a conductor wireline in the drilling pipe, the wireline having to be wound up to add a new length of pipe, which is time consuming and also to enable the drill pipe to be rotating whilst drilling to minimise longitudinal friction and to better control weight on the bit. Both these specifications relate to the steering of a drill bit angled relative to the pipe centreline to maintain the angle such that the bit is steered in the desired direction against the tendency of the bit to wander, due to the reaction on the motor body of the motor driving the bit. U.S. Pat. No. 3,841,420 discloses a mud pressure operated hydraulic clutch and electrical operation thereof by a relay controlled by a measuring unit.